1. Technical Field
The present invention relates to machine tools and systems and methods for positioning machine tools, particularly friction stir welders.
2. Description of the Related Art
Friction stir welding is a method of joining work pieces using friction heat generated at adjacent portions of the work pieces to form a plasticized region which solidifies as a welding joint. The work pieces may consist of initially separate pieces, or an integral piece having a crack, gap, or other opening. Some aspects of stir welding and stir welding systems are taught in U.S. Pat. No. 5,460,317 issued Oct. 24, 1995, to Thomas et al.; U.S. Pat. No. 5,697,544 issued Dec. 16, 1997, to Wykes; U.S. Pat. No. 5,893,507 issued Apr. 13, 1999, to Ding et al.; and U.S. Pat. No. 6,199,745 B1 issued Mar. 13, 2001, to Campbell et al.
As shown in FIG. 1, a friction stir welding head 1 includes a probe such as a pin 2, which is inserted into the joint between the work pieces 3. Typically, the pin 2 spins to generate friction heat to form a plasticized region along the joint for welding. The friction stir welding head 1 also includes a shank, shoulder or spindle 4, that contacts an upper surface 5 of the work pieces 3 to retain the plasticized material within the joint. Typically, the spindle 4 spins to form a plasticized region at the upper surface 5 of the work pieces 3. In some embodiments, the pin 2 and the spindle 4 may spin independently of each other at different rotational velocities.
The work pieces 3 may be supported by a rigid table or back plate 6, such as a steel plate. The rigid back plate 6 provides support so that the work pieces 3 do not bend or deform under the load. To maximize the strength of the joint, the welded portion should extend through the entire thickness of the work pieces 3. To assure that the weld extends the entire thickness, sufficient friction heat must be generated between the upper and lower surfaces of the work pieces. The pin 2 may be retractable to prevent leaving a keyhole at the weld termination, and to accommodate work pieces of different thickness.
The work pieces 3 are restrained to prevent movement away from one another as the pin 2 passes through the work pieces 3. The welding process creates high normal and side forces at the pin 2 to achieve the desired magnitude of friction. Consequently, the pin 2, spindle 4 and/or head 1 must overcome large forces exerted through the work pieces 3. This is particularly true when the pin 2 changes its direction of travel with respect to the work pieces 3. The relative motion of the pin 2 and work pieces 3 are identified by opposed arrows 7, 8.
Also as shown in FIG. 1, the pin 2 may be slightly angled in the direction of travel 7,8 with respect to the work pieces 3. The angle 10 is typically denominated as the angle of attack 10. Maintaining the angle of attack 10 is particularly difficult where the pin 2 changes its direction of travel 7,8 with respect to the work pieces 3.
In one aspect, friction stir welding system includes a turntable support; a turntable mounted to the turntable support for pivotal movement about a yaw axis; a yaw actuator having an elongated arm, the elongated arm coupled to the turntable at a point spaced from the yaw axis; a head support mounted to the turntable for movement therewith; a friction stir welding head having a first end and a second end opposed to the first end, a probe mounted proximate the first end for rotation about a longitudinal axis, and a spindle mounted proximate the first end for rotation about the longitudinal axis, the friction stir welding head supported by the head support for pivotal movement about a pitch axis, the pitch axis spaced between the first end and the second end of the friction stir welding head and extending laterally with respect to the longitudinal axis and perpendicularly with respect to the yaw axis; and a pitch actuator having an elongated arm, the elongated arm coupled to the friction stir welding head at a point on the friction stir welding head longitudinally spaced from pitch axis. The friction stir welding system may further include a plunge actuator having an arm, a gear box and a servo motor, the arm coupled to the turntable support to provide linear movement of the turntable support along a roll axis, the roll axis being perpendicular to both the pitch axis and the yaw axis; means for driving the probe; means for driving the spindle; a carriage carrying the turntable support, the turntable, the yaw actuator, the head support, the friction stir welding head, the pitch actuator, the means for driving the probe, means for driving the spindle; a frame supporting the carriage for linear movement with respect thereto; and a base supporting the frame for linear movement with respect thereto in a direction generally perpendicular to the direction of movement of the carriage with respect to the frame.
In another aspect, a friction stir welding tool includes a head support; a friction stir welding head having a first end and a second end opposed to the first end, a probe mounted proximate the first end for rotation about a longitudinal axis, and a spindle mounted proximate the first end for rotation about the longitudinal axis, the friction stir welding head supported by the head support for pivotal movement about a pitch axis, the pitch axis spaced between the first end and the second end of the friction stir welding head and extending laterally with respect to the longitudinal axis; and a pitch actuator having an elongated arm, the elongated arm coupled to the friction stir welding head at a point on the friction stir welding head longitudinally spaced from pitch axis.
In a further aspect, a machine tool for operating on a work piece includes a work bit having a longitudinal axis; a motor for rotationally driving the work bit about the longitudinal axis; a pitch actuator coupled to control movement of the work bit about a pitch axis; a yaw actuator coupled to control movement of the work bit about a yaw axis, the yaw axis perpendicular to the pitch axis; and a controller controllingly coupled to the pitch and yaw actuators and programmed to maintain an angle between the longitudinal axis of the work bit and a direction of travel of the work bit relative to the work piece, as the direction of travel of the work bit relative to the work piece changes between a first direction and a second direction.
In yet a further aspect, a method of operating a stir welder includes advancing a probe in a first direction relative to a work piece; advancing the probe in a second direction relative to the work piece, the second direction different than the first direction; and maintaining an angle between a longitudinal axis of the probe and a direction of travel of the probe relative to the work piece, as the direction of travel of the probe changes between the first and the second directions.